This Project will evaluate the effectiveness of cytosine arabinoside for treatment of biopsy proven progressive multifocal leukoencephalopathy in subjects with human immunodeficiency virus infection. A three arm study comparing best possible anti-retroviral therapy with either intravenous ARA-C therapy administered through an Ommaya reservoir. Twenty patients will be randomized to each arm of the study. The primary endpoint is death within 24 weeks. Secondary end points will be neurologic deterioration as indicated by neurologic examination and deterioration in Karnofsky Performance Scores.